Patent Specification GB 553,955 discloses a bone-anchored hearing aid comprising a microphone, an amplifier, a magnet with a coil, and a ferromagnetic armature, hereafter referred to as “bone anchor”. The bone anchor is implanted in the bone structure of a hearing-impaired individual's head. The amplifier amplifies the microphone output signal and drives the coil with the amplified signal. The coil cooperates with the magnet and the ferromagnetic properties of the bone anchor to induce vibrations into the bone structure. The vibrations propagate from the bone anchor to the cochlea of the aided ear mainly through the bone structure.
Bone-anchored hearing aids like the one described above may be used by individuals having asymmetric monaural bone-conduction hearing thresholds, i.e. a substantially higher monaural bone-conduction hearing threshold on one ear (the “bad” ear) than on the other (the “good” ear). In this case it is known to locate the bone anchor and the microphone of the hearing aid close to the bad ear in order to improve the individual's abilities to hear with the bad ear and to hear sounds originating on the bad-ear side of the head.
In order to provide a satisfactory compensation of the hearing loss, all hearing aids, including bone-anchored hearing aids, must be fitted to the particular needs of the hearing-impaired individual. An important part of the fitting process is to specify how the hearing aid shall control the amplifier gain. Hearing aids typically execute various signal processing algorithms, which modify the amplifier gain dynamically, e.g. in order to compress received sounds or adapt to changing listening environments. Most hearing aids control the amplifier gain in dependence on a gain setting. The gain setting typically defines the amplifier gain to be used in a specific listening situation and for received sound signals with a specific level. The gain setting thus functions as a basis for the dynamic control. The gain setting is typically determined early in the fitting process, but may be adjusted further during subsequent portions of the fitting process, e.g. in order to compensate for individual preferences and/or for deviations from theoretical values initially relied upon.
Prior to prescribing a hearing aid, the type, the severeness and the cause of the hearing loss are usually investigated in an initial diagnostic phase. A typical task in the diagnostic phase is to measure monaural bone-conduction hearing thresholds. The measurement is performed individually for each of the individual's ears. Usually, a test signal is emitted by means of a test vibrator, which is temporarily held against the skin just behind the ear to be measured. The test vibrator induces vibrations through the skin and tissue into the bone structure, through which they propagate to the cochlea of the ear to be measured. The thresholds are obtained by varying the level and the frequency of the test signal and recording for each frequency, at which level the individual is just able to hear the test signal. In order to improve the diagnosis of the hearing loss and its causes, an airborne masking noise may be emitted into the respective other ear, so that the test signal is only audible in the ear closest to the test vibrator.
Bone-anchored hearing aids have hitherto typically been fitted to the bad ear of an individual by determining a gain setting for the hearing aid's amplifier in dependence on measured monaural bone-conduction hearing thresholds for the bad ear. However, induced vibrations intended for the bad ear propagate to the good ear as well, and it is a known problem that a bone-anchored hearing aid may produce undesirably high sound levels in the good ear after being fitted to the bad ear of an individual with asymmetric monaural bone-conduction hearing thresholds.
A known remedy for the above mentioned problem is to determine the gain setting in dependence on measured binaural bone-conduction hearing thresholds, i.e. hearing thresholds measured for both ears simultaneously. Binaural bone-conduction hearing thresholds are typically measured by inducing a test signal, i.e. vibrations, at different levels into the bone structure and recording the lower one of the levels at which the individual is able to hear the test signal in at least one of the ears. The test signal is typically induced directly into the bone structure by means of the implanted bone anchor of the hearing aid itself. However, measuring binaural bone-conduction hearing thresholds is time-consuming, both for the person performing the fitting, i.e. the hearing-care professional, and for the hearing-impaired individual, and since such measurements are typically not performed in the diagnostic phase, this remedy adds to the cost and inconvenience associated with fitting a bone-anchored hearing aid.
There is therefore a need for a method of determining a gain setting of a bone-anchored hearing aid, which method remedies the above mentioned problem without requiring the hearing-care professional to perform additional measurements. It is an object of the present invention to provide such a method.
It is a further object of the present invention to provide a system, which is adapted to determine a gain setting of a bone-anchored hearing aid.